I would like to draw attention to Daniël announcing support for federation in their upcoming v2.0 release of their forum software, with help of a NLnet grant.
There are now 3 forum softwares that are implementing federation: Flarum, nodeBB and Discourse (via The Pavilion). I sincerelly hope that we do not get 3 incompatible federated apps, and would like to point out to the Fediverse Enhancement Proposals process (FEP) as a means to align ActivityPub implementations.
The FEP process sees a lot of participation, with proposals being discussed on the SocialHub ActivityPub developer community, which in turn is liaisoned to the W3C SocialCG that has sprung into activity again.